The invention relates to a method for generating electrical and/or mechanical energy from at least a low-grade fuel, in which steam is formed in a closed circuit with the aid of heat originating from the low-grade fuel, the steam formed is expanded with work being performed, the expanded steam is condensed and the condensate is reconverted into steam.
In industry, an endeavour is made to cause the generation of mechanical and/or electrical energy from fuels with as high an efficiency as possible. On the other hand, the economics impose limits because the price of the final product is mainly the sum of capital cost and fuel cost.
A distinction should be made between high-grade fuels and low-grade fuels. In general, low-grade fuels yield a lower efficiency in the generation of energy than high-grade fuels, while the investments in the installation are usually higher in the case of low-grade fuels than in the case of high-grade fuels. The high-grade fuels include the fossil fuels, such as petroleum, coals and natural gas. Low-grade fuels are, for example, waste materials and, with the present state of the art, also nuclear fuels.
There is a finite reserve of fossil fuels such as petroleum, coals and natural gas, but they can be converted into mechanical and/or electrical energy at relatively low to moderate capital cost with a high efficiency.
On the other hand, the world reserves of nuclear fusion materials are much greater than those of the fossil fuels, but the conversion of nuclear fuels into electrical energy at present requires high to very high investments, while the conversion efficiency is lower than the conversion efficiency of fossil fuels.
Modern society produces a large quantity of waste materials, which, viewed calorifically, still have a reasonable energy potential. In the conversion of waste materials into energy, however, chemical impurities limit the maximum process temperature so that this limits the conversion efficiency, while the investments in the conversion installations prove to be high to very high.
With the present state of the art, only one route is actually open for generating mechanical and/or electrical energy from waste materials, namely forming steam in a steam boiler by burning the waste materials and allowing said steam to expand in a steam turbine. Waste materials generally contain plastics such as PVC, and hydrochloric acid (HCl) is liberated during burning. This substance may cause serious corrosion in the steam boiler, in particular in the hot parts such as the superheater. In order to avoid rapid corrosion of this component, the steam temperature is limited to approximately 400.degree. C. In addition, for combustion engineering reasons, the excess of air should be chosen higher than in the combustion of fossil fuels. This results in turn in a lower efficiency of the steam boiler, which also affects the efficiency of the entire installation disadvantageously. All this has, in turn, the consequence that the steam pressure at the inlet of the steam turbine has to be limited in order to avoid the percentage of moisture in the outlet from the steam turbine becoming unacceptably high. A percentage of moisture of more than 10 to 13% produces serious erosion phenomena in the final stage(s) of the steam turbine. In a cycle in which only waste materials are burned, the efficiency in the generation of electrical energy usually remains limited to approximately 25%. If the high to very high investment costs in the installation are compared with this, then it emerges very quickly that such a solution is unable or hardly able to compete with the generation of electrical energy in power stations which are fired with high-grade fuels such as natural gas, oil or coals.
In contrast to the installations fired with waste materials, the formation of steam in the steam-forming section of a nuclear power station with the aid of nuclear fuels takes place at an efficiency of virtually 100%. Because no corrosive combustion products are separated in this process and nuclear power stations are exclusively large-scale installations, many techniques are available for introducing process refinements in such installations. However, there is a serious restriction in the case of nuclear power stations, and in particular, the high heat flux which occurs in the reactor. With the present state of the art, this heat flux can only be moved by cooling with water under high pressure, or vaporizing water. Steam has a lower heat transfer coefficient than (vapourizing) water, as a result of which it is not particularly suitable to be used in a reactor as coolant. In the modern nuclear power stations, only saturated steam emerges from the steam-forming section of the reactor, and, after partial expansion in a steam turbine, this is again heated with live steam and then expanded further to condenser pressure. In spite of all the process refinements and the efficiency of virtually 100% in the steam-forming section of the installation, the total efficiency of the entire installation remains limited to 30 to 35%.